1. Field of the Invention
The present invention relates to motor vehicles, more particularly to those of them enjoying anti-lock brake system (ABS) and/or traction control system (ASR, for anti-slip regulation), and still more particularly to a bleed valve in a hydraulic control unit of such a system.
2. Description of the Related Art
ABS (and ASR) is a part of the standard equipment in medium-class and upper-class vehicles. These systems are becoming increasingly included in lower-category and low-cost cars.
ABS and their variations typically comprise a hydraulic control unit (HCU) that houses hydraulic components of the system. The HCU is connected to the vehicle brake system between its master cylinder and calipers of wheel brakes.
A conventional hydraulic brake system, a one-wheel circuit 10 of which is shown in FIG. 1, comprises a tandem master cylinder (TMC) common for all wheels (shown is an inlet 12 from TMC), a HCU 14, and a wheel caliper (shown in FIG. 1 is an outlet 16 to the wheel caliper). Depicted within dotted lines is a low-pressure portion 18 of the HCU 14. The portion 18 typically contains a low-pressure accumulator 20, an input portion 22 of a return pump 24, and associated lines 26 and 28. A motor 30 drives the return pump 24. Depicted in FIG. 1 are also a return pump outlet valve 32, a noise damper 34, and two electromagnet valves: a pressure buildup normally open (NO) valve 36 and a pressure reduction normally closed (NC) valve 38. The pressure buildup electromagnet NO valve 36 is disposed in a brake line 40 connecting the inlet 12 from TMC to the outlet 16. A check valve 42 is connected in parallel to the NO valve 36. The pressure reduction electromagnet NC valve 38 is placed in a return line 44 that eventually, through the return pump 24 reunites with a main brake line 46.
Prior to filling the system including the circuit 10 with brake fluid, a vacuum is applied at the TMC to remove air from the system. When the vacuum is applied to the HCU, a portion of the unit, namely the low-pressure circuit, is isolated and does not evacuate its air. Any air bubbles still present in the hydraulic medium may bring about such an elasticity of fluid columns formed by the medium that the brake pressures required in wheel brake cylinders are not brought about to the requisite level.
ABS/ASR brake systems have several NC electromagnet valves; the valves of return pumps used in these brake systems are normally closed as well. The problem therefore exists that pockets of air may be present downstream of the NC electromagnet valves, and also downstream of the return pump valves. The air in these pockets may possibly be removed only with difficulty using liquid.
It has been proposed that at least the NC valves be triggered electrically upon evacuation and prior to filling of the system, and thus opened. To do so, special plug connections must be used to supply current to these electromagnet valves. The connections of this kind are expensive and involve operating costs when used, let alone that they are also complicated to manufacture. For many applications, therefore, it is the customer's desire not to energize the NC valves during the evacuation/filling process because of those issues involved.
Alternatively, in order to evacuate the low-pressure circuit, a check valve must be installed between the low-pressure circuit and the main circuit. The check valve must be set to a low delta-pressure value for good evacuation, but this can lead to unwanted movement of the valve during normal braking and ultimately to the loss of brake performance.
The reason for that to occur lies in that when the pedal is allowed to return rapidly, normal braking can create a transient partial vacuum in the brake system. The vacuum forms in the brake system when the momentum of the fluid tends to push into the master cylinder when the pedal abruptly stops. The transient vacuum at the HCU can open the check valve, allowing the partial vacuum to become trapped in the low-pressure circuit. Over time, the trapped vacuum tends to pull ambient air into the low-pressure circuit of the HCU. When the HCU subsequently goes into an ABS mode, the trapped air can be pumped into the brake circuit causing reduced brake performance.
Accordingly, a need exists in the art to provide a check valve for evacuating the low-pressure circuit without energizing the NC valves and also without subsequent movement during normal braking.